In the near future, systems for optical space communications between satellites, as well as between satellites and ground stations, will constitute an important and, on board of the satellites, weight-saving complement of the existing microwave technology. So-called optical terminals consist of one or several telescopes, which limit the angular range of the field of vision of an optical receiver in the direction to a counterstation, and also provide a directional transmission of signals to be transmitted. In addition, several movable mirrors are provided, by means of which the alignment of the transmitting and receiving direction is performed. Besides the direct detection of the optical output of the transmitter of the counter-station as the transmission method, the coherent superposition of the received light on the light of the same frequency from a local oscillator laser plays an important role since, besides great sensitivity to the signal to be detected, the insensitivity regarding interferences by background radiation is important.
Coherent methods need an additional local oscillator laser, on whose light the received light is superimposed on the photo detector of the receiver. Several types of lasers are available for all these purposes. Gas lasers offer the advantage of emitting light on only one optical frequency because of their so-called homogeneous spectral spreading of their optical output, although without any special measures, resonators of lasers have resonances on a whole series of equidistant frequencies. But this type of laser has a completely inadequate service life and dependability for applications in space. The diode lasers, which have achieved a high degree of development on account of their extensive employment in fiber-optical communications, represent an alternative at least for simple systems operating with intensity modulation and in addition are space-and weight-saving. But in spite of the operation on only one optical frequency, which has been achieved here also in the meantime, they are not yet generally suitable for coherent transmission methods with phase modulation, aside from complicated structures with large additionally coupled resonators. The reason lies in the still too large spectral width of this one transmitted optical frequency. Although fiber-optical coherent transmission systems also operate with commercially available diode lasers, detection is performed with a relatively large optical output because of the line-guided transmission, wherein in addition the use is limited to frequency modulation and differential phase modulation. In connection with the latter type of modulation, the change of the binary state of a data signal to be transmitted is transmitted by means of a modulated phase jump by 180 degrees of the transmitted light. The light beam is divided into two parts in the optical receiver and, with a mutual time displacement of the length of a data symbol, is sent to a common photodiode. Thus the light contains its own local oscillator, wherein the advantage of this method consists in that the phase of the unmodulated light used only needs to be stable within the framework of the length of a data symbol.
However, the interfering background radiation present in space, as well as the generally very low strength of the received signal, require an optical bandwidth of the unmodulated signal,which is considerably less than the modulation bandwidth. These are criteria which, together with small size and low weight, can best be met by diode laser-pumped solid-state lasers. Existing attempts to integrate the laser systems required for operation into a terminal for optical space communications have been described by Carlson et al. as well as Marshalek et al. (R. T. Carlson et al., "Monolithic Glass Block Lasercom Terminal: Hardware Proof of Concept and Test Results", SPIE vol. 2381, Free-space Laser Communication Technologies VII, February 7-8, 1995, San Jose, Calif., pp. 90-102: R. G. Marshalek et al., "Lightweight, High-Data-Rate Laser Communications Terminal for Low-Earth-Orbit Satellite Constellations", SPIE vol. 2381, Free-space Laser Communication Technologies VII, Feb. 7-8, 1995, San Jose, Calif. pp. 72-82).
Both groups of authors describe laser systems which are mechanically coupled to the optical device of the terminal and conduct their light emission by means of collimated beams. However, diode lasers have always been employed here. Diode laser-pumped solid-state lasers have a large volume and lesser efficiency, thus generate a larger amount of waste heat than comparable diode lasers. The increased amounts of heat produced in the vicinity of the optical system has been shown to be a risk for trouble-free operation of the optical devices. The unsatisfactory modulation capability of diode-pumped solid-state lasers represents a further problem. In contrast with diode lasers, the medium generating the optical output remains relatively long in an excited state after the pump energy has been supplied. Furthermore, the resonator of such lasers is considerably larger than that of diode lasers. Accordingly, limit frequencies of approximately 100 kHz are the rule for amplitude modulation, for example. The external modulation thus required is also very difficult to perform, since a high optical output must be manipulated, which requires the use of electro-optical modulators of low limit frequencies.
External modulation of laser light can be performed at high limit frequencies in modulators, in which the light is conducted through a waveguide, which permits a close mutual distance between the electrodes conducting the modulating voltage, and therefore a reduced modulation voltage. Since, because of the large magnification of the optical intensity caused by the narrow cross section of the optical waveguide, this method only permits low optical output, the modulated optical signal must be post-amplified. Attempts to do this consist, for one, in taking over methods and devices which, in the meantime, have proven themselves in fiber-guided optical communication, for example by post-amplification of the modulated optical system by means of an erbium-doped fiber amplifier (T. Araki, M. Yajima, S. Nakamori, Y. Hisada. "Laser Transmitter Systems for High-Data-Rate Optical Inter-Orbit Communications". Free Space Laser Communications Technologies VII, Feb. 7-8, 1995, San Jose. Calif. pp. 264-272).
It is also possible to derive corresponding traveling wave lasers from diode laser-pumped solid-state lasers, wherein suitable devices are available especially for the post-amplification of light from lasers operating with the same technology, particularly for the diode laser-pumped neodymium-YAG solid-state lasers, which are very useful for optical space communications because of their narrow spectral width.
To achieve great amplification with at the same time low optical pumping output, the light to be amplified must be conducted on as many paths as possible through the zone of an amplifying medium radiated by the pumping light. Because of this, with a respectively constant volume density of excited atoms for each coupled-in photon of the light to be amplified, there is a multiple of the probability of generating additional photons corresponding to the number of passages. In spite of low pumping output it is therefore possible to achieve an astonishing amplification factor. However, the devices corresponding to the prior art are constructed from several elements requiring a lot of space and mass, which therefore only poorly meet space travel-specific requirements. Even special developments contain the risk of insufficient mechanical load-carrying ability (T. J. Kane, E. A. P. Cheng, B. Nguyen, "Diode-Pumped ND:YAG Amplifier with 52-dB Gain", SPIE vol. 2381, Free-space Laser Communication Technologies VII, Feb. 7-8, 1995, San Jose, Calif., pp. 273-284; T. E. Olson, T. J. Kane, W. M. Grossmann, H. Plaessmann, "Multipass Diode-Pumped NF:YAG Optical Amplifiers at 1.06 .mu.m", Optics Letters, vol. 6, no. 5, May 1994, pp. 605-608). An additional problem for space travel applications consists in that the diode lasers used for generating the pump light also have a limited service life. It is accordingly necessary to maintain several redundant diode lasers for each diode laser-pumped solid-state laser and each diode laser-pumped optical amplifier in order to be able to replace outages. Several arrangements are known, wherein semiconductor structures, which allow a high optical output strength, are used for direct amplification by means of optical semiconductor amplifiers. For example, optical semiconductor amplifiers are described which have optical waveguides which, because of their extension laterally in respect to the spreading direction along the semiconductor junction, can conduct several modes of the light to be amplified along an electrically pumped semiconductor junction which generates an optical output. In accordance with U.S. Pat. No. 5,539,571, the exact control of the current flowing through the semiconductor still requires a high light output of the almost diffraction-limited strongly astigmatic light beam leaving the semiconductor chip. The amplifier can contain a broad. multimode, rectangular optical waveguide which, however, can also be designed trapezoidal to adapt itself to the path of the light caused by diffraction. Making use of the special shape of these waveguides, it is possible to realize lasers by means of this, which contain an unstable resonator (U.S. Pat. No. 5,392,308), are particularly stable in respect to the optical frequency of their emissions (U.S. Pat. No. 5,537,432), or which can also be coupled to external resonators (U.S. Pat. No. 5,499,261). Corresponding lens systems for forming focused or collimated light beams from the divergent, strongly astigmatic light beams of such amplifiers are disclosed in U.S. Pat. No. 5,321,718.
Thus, considerable improvements can be attained by means of using the prior art in respect to optical semiconductor amplifiers in comparison with existing attempts of using diode laser-pumped solid-state amplifiers.